The invention concerns a nuclear magnetic resonance (NMR) spectrometer for the measurement of fluid, in particular biological, and most importantly water containing samples with a magnet system for the production of a homogeneous static magnetic field B.sub.0 and with a probe head which exhibits an upper and a lower support, a connection for a feed conduit for the feeding of a fluid sample into the spectrometer, a connection for a drain conduit for draining the fluid sample out of the spectrometer, an in particular cylindrically shaped sample tube arranged between the upper and the lower support for the acceptance of the fluid sample, whereby the one end of the sample tube communicates with the connector for the feed conduit and the other end with the connector for the drain conduit, as well as a radio frequency (RF) coil, surrounding the sample tube, for the production and/or detection of an RF-magnetic field perpendicular to the direction of the static magnetic field B.sub.0 in a measurement volume within the sample tube.
An NMR spectrometer of this kind is, for example, known from DE-41 01 473 A1.
In NMR spectroscopic measurements of liquids it is usual to initially transfer the fluid to be investigated, which is normally stored and transported in a standard container, into a standardized glass tube which can be introduced into the probe head of an NMR spectrometer. In order to facilitate a quantitative comparison of the line intensity of different chemical components of the material being investigated, a reference fluid is often added to the material under investigation. In addition, for the purpose of stabilizing the homogeneous static magnetic field (or the measurement frequency) a deuterated fluid ("locking substance") is added, with the NMR signal of which, the magnetic field or the measurement frequency is stabilized ("locked"). The standardized NMR glass tube is subsequently introduced into an automatic sample changer from which it is introduced into the probe head at an appropriate time for taking of an NMR spectrum, usually with the suppression of the H.sub.2 O-line.
This method which, for example is known from the textbook "Practical NMR spectroscopy" from M. L. Martin et al., Heyden & Son Ltd., London 1980, in particular sides 365 ff, has the disadvantage that, in principal, a calibration fluid consisting of a reference fluid and/or a deuterated locking substance, must be added to the fluid material being investigated. This can be highly undesirable in particular for samples from the medical field (for example body fluids, cell- extracts and the like) or from food science chemistry (wine-extracts, milk, honey and the like). On the one hand the calibration fluids could undergo chemical reactions with the sample fluid and on the other hand, for example, the addition of a locking substance could change the pH-value of the fluid material under investigation and thereby lead to a changed NMR spectrum. The addition of D.sub.2 O to H.sub.2 O leads, furthermore, to a widening of the line due to the exchange between protons and deuterons. Thereby, in particular, the H.sub.2 O signal is widened which renders a suppression of the usually undesired liquid sample water signal more difficult since signal suppression is easier the sharper the corresponding measured line.
Especially with medical samples, the addition of a standard liquid to the sample substance is likewise critical since the substantive content of the sample fluid can render the standardization more complex. This leads, on the other hand, to a change in the line width of the standard which renders a quantitative comparison of the line intensities more difficult.
A further disadvantage of the classical procedure is that the liquid sample, after the addition of the calibration fluid, can usually no longer be separated from the latter so that the sample substance is, possibly, unusable for further investigations and tests following the NMR measurement.
Finally, it is also been shown to be disadvantageous for the liquid sample be prepared prior to each NMR measurement through the addition of locking substances and/or standardized fluids, a process which usually must be carried out manually and, in addition to a substantial time loss, also encompasses certain risks such as, e.g., improper technical handling or shaking of the sample. In particular with the serial measurement of a plurality of similar fluid samples the above mentioned procedure is cumbersome and detrimental to an automatization of the measurement procedure.
Problems similar to those of the batch-procedure described above in NMR spectroscopic measurement of fluid samples result in the known flow-through method. A method and an apparatus for coupled fluid chromatography and NMR measurement, is, for example, disclosed in DE 41 04 075 C1, with which certain volumes of sample fluids are sectionwise introduced to the NMR probe head using the stop-flow technique. With the apparatus therein described it is, however, not possible to measure a mixture of fluid samples, rather the components of the fluid sample are chromatographically separated prior to the NMR spectroscopic measurement.
A continuous flow-through cell with which fluid mixtures can be subjected to an NMR measurement is, for example, known from U.S. Pat. No. 4,775,836 where a spherically shaped measurement cell, in the homogeneous center of an NMR spectrometer is described through which liquid sample substance is continuously fed under high pressure. An advantage of this configuration is that, through the spherical configuration of the measurement volume, artifacts due to inhomogeneities of the magnetic susceptibility in the measurement volume can be eliminated. Nevertheless, also in this case, the locking substance and/or standard liquid must be mixed with the sample fluid if a frequency stabilization, a field stabilization, or a quantitative comparison of the line intensities be required.
The same is true for the method known from DE 41 01 473 A1, and the accompanying NMR spectrometer likewise described in this publication, which exhibits all the categorizing features mentioned above.
Glass tubes which are insertable within another have been known for a long time in NMR spectroscopic applications with which one tube is filled with the sample fluid and the other with a calibrating fluid, the tubes then being introduced together into the probe head of the NMR spectrometer for measuring purposes (see for example catalog 5/92-7 of Wilmad Glass Company, Buena, N.J. 08310 USA, pages 28 and 29). In this manner, although a mixing of the material being investigated with other materials from the calibration liquid is prevented, the required manual filling procedure of both fluids, the putting together of the tubes, and the introduction into the spectrometer continue, however, to be very time consuming and still bear the risks of a shaking or an unintentional mixture of the fluids involved. The time delay thereby produced between the extraction of a medical sample fluid and the NMR measurement can be very disadvantageous for the investigation of sensitive and easily disrupted body fluids. Furthermore, there is increased danger during the filling-in and filling-up procedure of inducing a contamination of the liquid sample with bacteria, air, dirt etc. Through a differing individual treatment of the manually prepared sample fluids, measurement errors in serial measurements cannot be ruled out with sufficient certainty. In addition, with the utilization of the known double tube, the application of a flow-through probe head is ruled out, e.g., with an apparatus with which such double tubes are utilized, it is only possible to carry out fluid measurements in a sectionwise fashion.
It is therefore the underlying purpose of the invention to introduce an NMR spectrometer of the above mentioned kind for measurements of fluid samples with which the sample fluids can be measured without additives and, after the measurement, be recaptured in their original state with which a special prehandling of the sample is unnecessary. Accordingly the inventive spectrometer can be utilized for flow-through measurements as well as in sectionwise operation (stop-flow) without substantial reconfiguration and is suitable, in particular, for automatic serial measurements of a plurality of similar fluid samples, whereby, on the one hand, little or no manual preparation of the sample should be necessary, and on the other hand a field stabilization and/or a quantitative comparison of the line intensities on the basis of a reference substance can be accommodated.